This invention is in the field of mobile wireless communications, and is more specifically directed to the initiation of multiple access communications sessions.
The popularity of mobile wireless communications has increased dramatically over recent years. It is expected that this technology will become even more popular in the foreseeable future, both in modern urban settings and also in rural or developing regions that are not well served by line-based telephone systems. This increasing wireless traffic strains the available communications bandwidth for a given level of system infrastructure. As a result, there is substantial interest in increasing bandwidth utilization of wireless communications system to handle this growth in traffic.
Modern digital communications technology utilizes multiple-access techniques to increase bandwidth utilization, and thus to carry more wireless traffic. Under current approaches, both time division multiple access (TDMA) and code division multiple access (CDMA) techniques are used in the art to enable the simultaneous operation of multiple communications conversations, or wireless “connections”. For purposes of this description, the term “conversations” refers to either voice communications, data communications, or any type of digital communications. As evident from the name, TDMA communications are performed by the assignment of time slots to each of multiple communications, with each conversation transmitted alternately over short time periods. CDMA technology, on the other hand, permits multiple communications sessions to be transmitted simultaneously in both time and frequency, by modulating the signal with a specified code. On receipt, application of the code will recover the corresponding conversation, to the exclusion of the other simultaneously received conversations.
As is fundamental in the art, a single base station in a wireless communications network conducts communications sessions with multiple mobile wireless transmissions in an area of coverage, or “cell”. In addition, each base station is aware of the remaining bandwidth available for new communications sessions that may be initiated relative to a wireless unit within its cell. In this regard, the base station is aware of the presence of those mobile wireless units that are turned on and within its cell, and also of the identity of those units, regardless of whether the units are currently connected in a conversation. In this way, wireless units may be called by another party from anywhere in the telephone network, and the wireless units themselves may initiate a connection by placing a call.
In order for a wireless unit to place a call to a particular telephone number, it must send a request for a connection to the base station. An initiation sequence is then carried out, according to conventional systems, in which the channel for the desired communications is assigned by the base station and acknowledged by the wireless unit.
For example, in a CDMA system, the base station and wireless unit must “agree” upon a modulation code to be used in the communications link between these two stations. In conventional CDMA systems, the codes are not determined a priori, given the transient nature of wireless units within a base station coverage area. As such, techniques have been developed by way of which the wireless units and base station may communicate prior to the assignment of a modulation code. According to a widely used technique for this initialization, the base station periodically broadcasts signals that indicate the number and position of reserved time slots within a communications frame for initialization, to each of the wireless units in its area that are not currently connected. These broadcast signals are received by each wireless unit, so that, in one of these time slots, the unit may send a signal to the base station to request a connection. This request signal is commonly referred to as a “preamble,” following which the message part of the transmission is communicated.
It is quite likely, however, that multiple wireless units may try to establish communications at the same time, and may therefore be simultaneously sending preambles within the same time slot. As such, conventional CDMA wireless communications systems specify a set of modulation codes from which the wireless unit selects a code to request a connection. The codes in the set are orthogonal relative to one another, in the sense that the base station can resolve the sources of simultaneously received messages encoded by different ones of the set of modulation codes. Because the requesting wireless unit typically selects a modulation code in a pseudo-random manner, these channel selection codes are typically referred to as “random access” codes. These random access codes greatly reduce the probability of a collision between two (or more) wireless units in a coverage area requesting a connection at the same time slot. For example, if eight time slots are available for requesting a connection, using one of sixteen available random access codes, the likelihood of a collision between two wireless units that request a connection is reduced from one in eight to one in 128.
An example of this random access approach uses a 256 chip spreading code in the generation of the preamble part of the transmission. This conventional approach is described in Technical Specification TS 25.213 V2.1.0: Spreading and Modulation (3rd Generation Partnership Project, 1999). To request a communications session according to this approach, a wireless unit randomly selects one of sixteen signature symbols for its preamble. The signature consists of a sixteen-symbol sequence of plus or minus the complex value A=1+j. One example of a sixteen symbol signature is [A, A, A, −A, −A, −A, A, −A, −A, A, A, −A, A, −A A A]. Each symbol in this preamble is then spread into 256 consecutive chips, following which the spread preamble is modulated and transmitted to the base station by the requesting wireless unit.
The mobile nature of the wireless units presents certain difficulties to the resolution of simultaneous encoded request signals, however. Although random access codes, such as the 256-chip spread coded random access preamble noted above, provide signatures that are theoretically orthogonal, this orthogonality presumes simultaneous receipt at the base station. As noted above, preambles are simultaneously transmitted by mobile units in the time slots specified by the base station. However, simultaneously transmitted preambles from widely differing distances in the cell will not simultaneously arrive at the base station. According to the conventional 256-chip spread coded approach, coded signatures are not necessarily orthogonal when one preamble is significantly time-shifted relative to another. In other words, time-shifted preambles coded according to this conventional approach will cross-correlate with one another. As such, in some circumstances, conventional CDMA base stations may not always be able to resolve different random access codes from multiple wireless units.
This cross-correlation of random access codes received from varying transmission distances has been addressed by prior techniques. For example, a so-called “long” code has been developed which uses a real-valued version of the uplink spreading code to spread the wireless unit signature over a much longer preamble. The length of the preamble is, in this approach, selected to be significantly longer than the greatest time delay expected within a given cell. This long code is derived simply by spreading each bit of a sixteen-bit Gold code signature symbol A over a number of chips, for example 256 chips; in this case, the sixteen-bit symbol becomes sixteen sequences of 256-chip values, for a total length of 4096 chips. This longer preamble greatly reduces the cross-correlation between orthogonal signatures that are received at the maximum delay (and thus the maximum differential distance) relative to one another.
However, it has been observed that this long code approach remains vulnerable to velocity variations between requesting mobile wireless units. The well-known Doppler effect refers to the shift in frequency that results for a moving source of periodic signals. For the case of mobile wireless units in a moving automobile, train, or especially an airplane, the Doppler shift causes a phase shift that accumulates over the transmission length of the request. As noted above, the conventional “long” random access code has a length of 4096 chips (i.e., sixteen symbols of 256 chips each), over which the orthogonal signatures are analyzed to resolve different wireless units. Because of this code length, the accumulated Doppler phase shift can cause cross-correlation among codes, so that the base station may not be able to resolve simultaneous transmission requests.
Other approaches for encoding random access channel preambles have been derived to address the problem of Doppler shifts on the transmitted signals. One approach utilizes a differential encoding technique, in which the signature is determined by the differences between adjacent symbols in the preamble. Some level of cross-correlation for time-delayed signals has been observed for this differential approach, rendering it somewhat vulnerable to differences in distance between simultaneously-transmitting mobile wireless units. Because of this vulnerability, coherent encoding over a long (e.g., 4096 chip) preamble has been used for slowly moving or stationary transmitters to provide adequate orthogonality for variations in transmission distance, while rapidly moving mobile units utilize the differential coding. Of course, the implementation of different random access channel encoding for mobile units of different velocities significantly increases the complexity of transmitters and base stations.
Another approach uses segmented non-coherent decoding for fast-moving transmitters, in which the receiver decodes the preamble in shorter segments of symbols, for example four segments of four symbols each. According to this technique, however, the segments are not orthogonal relative to one another.